In exposure apparatus used in the manufacture of semiconductor devices, a photolithography process is used wherein a circuit pattern formed on an object, like a reticle or mask (generically referred to hereinafter as a "mask") is illuminated by an illumination optical system. An image of this pattern is formed and transferred by a projection optical system (i.e., a projection lens) onto a photosensitive substrate like a glass plate or wafer (generically referred to hereinafter as a "wafer") coated with a photosensitive agent like photoresist. However, it is necessary to set the intensity of the light beam from a light source provided in the illumination optical system to achieve a proper exposure level (defined as the light intensity multiplied by the time of exposure), based on the photosensitivity of the photoresist on the wafer. One method of controlling the intensity of light beam from the light source is by arranging an intensity controlling means, such as a light-attenuating member (e.g., a filter) in the optical path of the illumination optical system.
In conventional illumination optical systems, a light intensity controlling system, such as a light-attenuating filter, is arranged in the optical path removed from the light source. In this case, the light beam from the light source will directly impinge upon one or more optical elements arranged between the light source and the filter without being attenuated. Moreover, in conventional illumination optical systems, a large number of high-intensity light beams irradiate the one or more optical elements (e.g., lenses) on the light-source-side of the filter.
Unfortunately, conventional illumination optical systems as described above are not suitable for certain present-day applications. For example, in semiconductor manufacturing using deep ultraviolet (DUV) light, the optical elements (e.g., lenses) have the characteristic that their light transmittance changes with the amount of electromagnetic waves (e.g., light waves) to which the lens is exposed. The degree of change in transmittance depends on the composition of the lens and the wavelength and intensity of the electromagnetic waves. The greater the intensity of the electromagnetic waves, or the greater the amount of irradiation, the more the light transmittance of the lens changes. Also, depending on the composition of the lens, irradiation by electromagnetic waves causes a chemical reaction, wherein various ions, such as Na.sup.+, diffuse out of the lens surface. This results in so-called fogging that irregularly reflects light impinging on the lens. This, in turn, results in markedly deteriorated lens performance and a decline in lens durability.